Watermelon [Citrullus lanatus (Thunb.) Matsum.
& Nakai] production in the United States
has, in the past few years, incurred significant
losses to races 1W and 2WU.S. powdery mildew
(Podosphaera xanthii) infection. We report the
mode of inheritance, gene action, and heritability
of race 2WU.S. resistance in two populations
derived from a cross involving the resistant genotype
PI 189225 and the susceptible ‘Charleston
Gray’ and PI 269677. Parents, F1, F2, BC1P1,
and BC1P2 plants were inoculated and evaluated
for leaf and stem resistance in two replicated
greenhouse experiments. Segregation
patterns revealed that only leaf resistance rating
in Charleston Gray × PI 189225 fit the model
for single gene inheritance. Generation mean
analysis established only additive gene action
for leaf resistance while for stem resistance,
additive, dominance, and epistatic effects were
important. Narrow-sense heritability estimates
were higher for stem (0.81) than for leaf (0.58)
resistance. Lack of dominance and epistatic
effects combined with high heritability indicated
high probability of success in selecting for leaf
resistance in early generations. Stem resistance
had a higher additive effect, lower dominance,
and moderate heritability, but progress toward
resistance should be possible. In population
PI 269677 × PI 189225, epistatic effects combined
with low heritability (0.20) and presence
of duplicate epistasis may result in slower progress
from selection.